The concept Lab-on-a-chip (LOC) emerged at the beginning of 1990's. Three phases of a biomedical assay are incorporated into LOC devices, i.e. sample pre-treatment, biochemical reaction, and signal detection. Lab-on-chip microsystems may have the following advantages:                They require much smaller sample quantities than traditional wet-bench laboratory work.        Many biochemical reactions can take place in parallel with high automation and reproducibility.        The increased dynamic chemical performance due to the increased surface-to-volume ratio in microsystems speeds up the bio-assay process to a great extent.        As the biochemical reactions perform in a closed system without direct manual operations, contamination and uncertainty can be reduced.        
However, scaling down such LOC systems may not be straightforward. One of the new challenges is the transport of the sample (bio-analytes, e.g. cells or bio-molecules, in aqueous buffer) between different functional compartments of the system. In microsystems, it is more difficult to carry the bio-analytes simply by a fluid flow because traditional actuation forces (e.g. mechanical force, electro-osmotic force, acoustic force) significantly decrease as the system feature sizes scale down. As a result, the active actuation forces become less important when compared to resistive forces (e.g. surface tension) or fluctuations in the system.
Magnetic particles may be used in lab-on-a-chip systems for cell separation, magnetic bio-assay, and other applications. Target bio-analytes (e.g. bio-molecules or cells) can be specifically captured by functionalized magnetic particles and then be attracted or transported by on-chip electrically controllable electromagnetic fields.
An alternative method for sample transfer is to transport the bio-analytes without moving the fluid. This can be achieved by different approaches such as dielectrophoresis and magnetophoresis.
Dielectrophoresis (DEP) is a very effective method for particle manipulation and separation. This technique is usually applied to cells, cell organelles or other particles (e.g. cell content and its membrane). If a particle is subjected to an electric field, charges will be induced due to the relative permittivity and conductivity of the particle when compared to the medium. This process is called polarization. The particle can be driven by the electrostatic force if the external electric field is non-uniform. Particularly in an AC electric field, the particle polarization is frequency dependent, i.e. the polarity and strength can be adjusted by changing the frequency and amplitude of the AC electric field. As a result, the induced force and hence the movement of the particle can be adjusted. This is called dielectrophoresis (DEP). By changing the induced force, the particle can be attracted or repelled by conventional DEP or moved bi-directionally by traveling wave DEP. DEP can also be used to identify or separate different particles (e.g. different types of bacterium, living or dead cells). The main advantage of DEP is that the actuating force, and hence the motion style, can be controlled by a simple electric field.
However, there are also disadvantages to DEP. The DEP performance is highly sensitive to the fluid, e.g. buffer, especially ion strength. A large DEP force can only be obtained in a medium with low ionic strength whereas the ionic strength of real samples such as e.g. blood is higher by several orders of magnitude. Furthermore, as the DEP force amplitude is roughly proportional to the volume of the particle, it is only suitable for the manipulation of large particles, e.g. cells, but it is too small for small molecules. In addition, the DEP of bio-analytes is a physical effect which does not necessarily reflect the biological property of the analyte. Therefore, it could be difficult to manipulate the analyte with certain specificity in a complicated environment.
There have been quite a few examples of DEP manipulation of bio-analytes. For example, different moieties in a medium can be separated from each other because of their different DEP properties (see, e.g., US 2003/047456, US 2004/653020, U.S. Pat. No. 6,858,439). By carefully selecting the DEP frequency, the target component can be trapped by a positive DEP force while all other components are not captured. Furthermore, traveling wave DEP can separate different moieties as well (U.S. Pat. No. 6,596,143, US 2001/045359).
Another method for bio-analyte transport is to use magnetic particles as carriers. Functionalized magnetic particles have been used for target bio-analyte separation for years. In microfluidic systems, magnetic particles can be actuated by a magnetic force. When the magnetic particles are attached to target bio-analytes, the bio-analytes can be transported together with the magnetic particles. This method is called magnetophoresis (MAP). Different approaches were reported to generate magnetic fields for particle transport.
The magnetic field can be applied by external magnets. When the fluid carries the magnetic particles, the magnetic particles bound to the bio-analyte will be attracted towards the magnet(s) and can be separated from other components in the medium. Particularly, by making use of different mobility of different magnetically labeled bio-analytes, the target bio-analytes can be separated from other components (see U.S. Pat. No. 6,467,630).
Alternatively, especially in microsystems, the magnetic field can be applied with microfabricated electromagnets (see US 2004/262210). In this case, the micro electromagnets are current-carrying micro-conductors. The current sent through these conductors generates a local magnetic field which is able to attract and/or continuously move the magnetic particles and, hence, the bio-analytes bound to the particles (see US 2002/166800, EP 1462174).
An advantage of MAP is the fact that it keeps the bio-specificity due to the bio-affinitive binding between the magnetic particle and the bio-analyte. Another advantage is that the magnetic force applied to the bio-analyte does not depend on the size of the analyte but is only determined by the magnetic particle and the applied magnetic field. Still another advantage is that the magnetic force is not affected by the medium as most media do not contain any magnetic component. Meanwhile, the possibility of integrating magnetic sensors, e.g. magnetoresistive sensors, in a microsystem can easily feature the system with detection functionality, which is very useful for lab-on-a-chip applications.
Despite these magnetic particle transport mechanisms, there is still a serious problem for transport of e.g. bio-analytes in particular applications. FIG. 1 schematically illustrates forces exerted to a magnetic particle M in a medium flowing over a substrate in a magnetic field. The forces experienced by the magnetic particle M are (1) a magnetic force (Fm), (2) a force (Ffl) exerted by the fluid on the magnetic particle M, (3) a Derjaguin-Landau-Verwey-Overbeek force (FDVLO) and (4) gravity (Fg). For inducing a magnetic field, a conductor 5 covered by a dielectric layer 6, also called passivation layer, may be included in the substrate. As most magnetic particles M for biological applications are super-paramagnetic or paramagnetic, the magnetic particles M move to the place where the magnetic field is stronger. Therefore, when the magnetic field is generated by an on-chip electromagnet, the magnetic force (1) (Fm) always attracts the magnetic particle M towards the substrate. Depending of the orientation of the substrate, also the gravity (4) (Fg) can attract the magnetic particle M towards the substrate. Meanwhile, if the magnetic particle M is close enough to the solid substrate, the Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction between the magnetic particle M and the substrate surface becomes significant. The DLVO interaction includes the effect of Van der Waals attraction and electrostatic interaction. The DLVO force (3) (FDVLO) can be attractive or repulsive depending on the material the magnetic particle M is formed of and the material of the substrate surface as well as the pH and ionic strength of the medium. If the DLVO force (3) (FDVLO) is repulsive and is large enough, it could balance the attractive out-of-plane component of the magnetic force (1) (Fm) so that the magnetic particle M is kept levitated in the medium. However, if the repulsive DLVO force (3) (FDVLO) is not strong enough or if the DLVO force (3) (FDVLO) is attractive, the magnetic particle M will be brought to the substrate surface by the sum of DLVO force (3) (FDVLO) and the magnetic force (1) (Fm) until it finally gets in contact with the substrate. Once the magnetic particle M adheres to the substrate surface, it becomes difficult to move the magnetic particle M by the magnetic field or the force exerted by the fluid on the magnetic particle M (2) (Ffl).
In order to avoid the adhesion problem, surfactants can be added to the medium in order to fully charge the surface of both magnetic particles M and the substrate surface. As a result, a large repulsive DLVO force (3) (FDVLO) can be obtained. However, the use of surfactants is rather restricted in practical biochemical reactions, especially with cells. In most biochemical operations, the DLVO force (3) (FDVLO) can be very small mainly due to the neutral pH and high ionic strength. In addition, it is not always opportune to change the medium arbitrarily and thus the DLVO force (3) (FDVLO) cannot be used to balance the attractive magnetic force (1) (Fm). This problem can seriously affect the application of magnetic particles M as bio-analyte carriers in lab-on-a-chip systems.
A more powerful but more complex approach could be the combination of different physical forces for bio-analyte manipulation. These forces can be DEP force, magnetic force and/or acoustic force.
The combination of a magnetic force and a negative dielectrophoretic force for selectively separating target bio-analytes with magnetic particles was described in WO 2001/96857 and is illustrated in FIG. 2. Fabricated magnetrodes 7 (micro-magnetic structures) apply magnetic forces to the magnetic particles M1 and M2 carried by the fluid. In the mean time, an AC electric field is also applied to the particles M1 and M2 by electrodes 8 on top of the magnetrodes 7 to induce a negative dielectrophoresis. The repulsive DEP force balances the attractive magnetic force at a certain separation distance (the distance between the particles M1 and M2 and the device). Consequently, magnetic particles M1 and M2 with different magnetic and DEP properties can be levitated at a different separation distance, and hence they can be separated from each other by the fluid flow. Although in this example the separation distance of the magnetic particles M1 and M2 can be controlled by the balance of the magnetic force and the DEP force, this approach is not capable of actively transporting the magnetic particles M1 and M2 by traveling micro-electromagnetic fields. Instead the magnetic particles M1 and M2 are still carried by the fluid. The magnetic force is applied on the magnetic particles M1 and M2 by pre-deposited magnetrodes 7 (in an external magnetic field when necessary).